Organic light-emitting diode (OLED) display devices utilize a current passed through thin-film layers of organic materials to generate light. Electrodes located on either side of the organic layers provide current to the organic layers. The color of the light emitted by a light emitting element depends on the specific organic material used to make the OLED. Alternatively, color display devices can be made using the organic materials that emit white light and an array of color filters (CFAs) is located over the array of light emitting elements.
It has been known for many years that the human eye is most sensitive to green light and less sensitive to red and blue light. More specifically, the spatial resolution of the human visual system is driven primarily by the luminance rather than the chrominance of a signal. Since green light provides the preponderance of luminance information in typical viewing environments the spatial resolution of the visual system during normal daylight viewing conditions is highest for green light, lower for red light, and even lower for blue light when viewing images generated by a typical color balanced image capture and display system. This fact has been used in a variety of ways to optimize the frequency response of imaging systems. For example, signals carrying color image information are sometimes designed with a luminance or green signal at one resolution and two color-difference signals at a lower resolution.
U.S. patent application Ser. No. 2002/0015110 by Brown Elliott, published Feb. 7, 2002, shows an arrangement of color pixels for a full color imaging device that takes into account the spatial response of the human visual system to different colors of light. Referring to FIG. 2, one example proposed by Brown Elliot includes a repeating rectangular pattern 10 of red 12 green 14 and blue 16 rectangular light emitting elements wherein pairs of red and green light emitting elements occupy diagonal corners and a blue light emitting element is located in the middle of the pattern.
It is known that the efficiency of light production by various OLED light emitters of different colors varies. OLED display devices also suffer from color-dependent age-related degradation in luminous efficiency. As the devices are used, the energy required to produce the various colors in the devices increases at different rates. This aging is dependent on the current density used to drive the organic materials. A larger current density causes faster aging; a smaller current density causes slower aging. Thus, over time, as OLED display devices are used at a constant current level, the displays grow dimmer and the white color produced by driving a combination of red, green and blue colored light emitting elements changes.
It has been proposed to adjust the relative sizes of the different colored light emitting elements to compensate for color-dependent age-related degradation in luminous efficiency. The development of OLED displays with different sizes of red, green, and blue light emitting elements is disclosed in U.S. Pat. No. 6,366,025, issued Apr. 2, 2002 to Yamada. Yamada discloses an OLED display with unequal light emitting element areas, wherein the area of the light emitting elements are adjusted with the goal of improving the lifetime of the OLED display. While Yamada discloses light emitting elements having different sizes for optimizing the lifetime of the display, the display is not optimized with respect to the response of the human visual system.
There is a need therefore for an improved OLED display that simultaneously optimizes the resolution and lifetime in an OLED display device.